Recently, because of its peculiar shape, a polymer brush is paid to attention. The polymer brush has a structure in which a polymer chain of which end is fixed (by chemical bond or adsorption) to the surface of solid is extended along a direction vertical to the surface of solid. The degree of extending of a polymer chain depends significantly on graft density.
The polymer brush is obtained usually by grafting a polymer chain to the surface of solid by surface graft polymerization, particularly, surface initiation living radical polymerization.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2001-131208 discloses a process of preparing a polymerizable brush base material comprising a process of providing a base material carrying at least one covalently bonded free radical initiator having a radical generating portion at a position distant from the base material, and a process of contacting the covalently bonded base material with a monomer, under conditions of promoting free radical polymerization from the radical generating portion of the initiator, to form a polymerizable brush.
JP-A No. 2002-145971 describes a process for producing a polymer brush by surface initiation living radical polymerization. The surface initiation living radical polymerization is specifically a method in which, first, a polymerization initiator is fixed by a Langmuir-Blodgett (LB) method or a chemical adsorption method to the surface of solid, then, a polymer chain (graft chain) is grown on the surface of solid by living radical polymerization (ATRP method). This publication describes that growth of a polymer chain of regulated length and length distribution on the surface of a substrate at high surface density conventionally not found is made possible by surface initiation living radical polymerization, and due to its high graft density, membrane thickness matching even elongated chain length is obtained by swelling with a solvent, realizing a condition of “polymer brush” in the true sense for the first time. This publication also describes that in conventional radical polymerization by surface initiation, a radical once generated grows until irreversible stopping to generate graft chains sequentially, thereby preventing adjacent grafting because of steric hindrance of a previously grown graft chain, while in this system, polymerization progresses in living mode, namely, all graft chains grow approximately evenly, thereby, steric hindrance between adjacent graft chains is decreased and, this is believed to be a factor for obtaining high graft density.
The above-mentioned JP-A No. 2002-145971 discloses a nano structure functional body characterized in that graft polymer chains constituting a graft polymer layer disposed on the surface of a substrate by graft polymerization obtained by such surface initiation living radical polymerization has a structure multi-layered in chemical composition along membrane thickness direction by copolymerization with other monomers or oligomers. Further, this publication discloses also a nano structure functional body characterized in that a polymerization initiation portion (polymerization initiation moiety) of molecules disposed on the surface of a substrate is inactivated in given pattern along membrane surface direction, then, a polymerization initiation portion not inactivated is graft-polymerized to give a graft polymer layer disposed in given pattern.
In addition, Tsujii Takanori, “Polymer Brush no Shintenkai”, “Future Material”, vol. 3, no. 2, p. 48 to 55 also describes in detail a high density (dense) polymer brush obtained by surface initiation living radical polymerization.
Regarding the polymer brush, applications to various uses have been investigated as described below.
The above-mentioned JP-A No. 2001-131208 describes that the resulting polymer brush is useful in solid phase synthesis of an array of peptides, polynucleotides or organic lower molecules.
The above-mentioned JP-A No. 2002-145971 describes that the disclosed nano structure functional body is useful as a complex particle, complex element, multi-functional sensor or the like showing responsibility against outer stimulation.
JP-A No. 2001-158813 describes application of a polymer brush to surface modification of a contact lens, intraocular lens, artificial cornea and the like. Further, this publication also describes application of a polymer brush to a dialyzer for kidney, blood reservoir bag, conductive wire of a pace maker, blood vessel transplant, bandage for injury therapy, eye patch, drug delivery patch, cardiac bulb, blood vessel for transplantation, catheter, artificial organ and Langerhans island.
Japanese Patent Application National Publication (Laid-Open) No. 2002-504842 describes application of a polymer brush to a stent.
Japanese Patent Application National Publication (Laid-Open) No. 2002-535450 describes application of a polymer brush to a nucleic acid molecule detecting method (DNA sensor and the like) and a method of purifying a compound such as nucleic acids, (poly)saccharides or (poly)peptides, or their complexes, antibodies and the like from a sample. Further, the above-mentioned publication describes also use of a polymer brush as an affinity matrix, its use as a sensor chip, its use for fixing of an initiation molecule for formation of an oligomer or polymer, preferably, for synthesis of a nucleic acid or peptide, and its use as a gel in separation of molecules, preferably, organism molecules, in electric field.
Though differing from a polymer brush, A. K. GEIM et al., “Microfabricated adhesive mimicking gecko food-hair”, Nature materials, Vol. 2, July 2003, p. 461-463 describes a high density array of a polyimide in the form of pyramid (hair). Specifically, a polyimide film having a thickness of 5 μm is formed on a silicon base plate, and an aluminum pattern is transferred to the polyimide film by oxygen plasma etching using an aluminum mask, to form, for example, a polyimide array in the form of pyramid having a diameter of 0.6 μm and a height of 2.0 μm. This publication describes also that this has high stickiness.
These conventional polymer brushes are capable of having a structure of maximum elongation of a polymer chain (graft chain) only in a good solvent, and under dry condition or in a poor solvent, have a structure of a fallen or folded polymer chain (graft chain).
The supercritical fluid is a fluid of which density is near that of liquid and of which viscosity and diffusion coefficient are near those of gas, and has diffusibility of gas and substance dissolvability of liquid, together. That is, the supercritical fluid has various effects as a reaction solvent.
Conventionally, the supercritical fluid is utilized for separation by extraction of active ingredients, removal by extraction of unnecessary components, and the like such as extraction of hop extracts and aromatics, decaffeination from coffee and tobacco, and the like. For example, production of caffeine-less coffee utilizing supercritical carbon dioxide has been industrially carried out from approximately the latter half of the 1970's.
Recently, the supercritical fluid is utilized also for removal and concentration of impurities such as chemical raw materials, products and the like such as removal of unreacted monomers from a polymer, concentration and dehydration of alcohol, and the like. Further, the supercritical fluid is utilized also for de-bindering of ceramics, washing and drying of semiconductors and machine parts, and the like. For example, JP-A No. 7-149721 discloses a method of purifying a bismaleimide compound characterized in that an ether imide-based bismaleimide compound containing impurities such as aromatic hydrocarbon solvents and the like used in production is subjected to impurity extraction removal treatment of contacting with carbon dioxide under supercritical condition including a pressure of 60 atom or more and a temperature of 20° C. or higher or under condition near the supercritical condition.
Additionally, the supercritical fluid is utilized for fine particle formation, thin film formation and fine fiber formation by rapid expansion (RESS method) such as production of barba-like fine particles such as silica and the like, and also for fine particle formation and thin film formation by poor solvent achievement (GAS method) such as reinforcement (surface coating) of silica aero gel, and the like. For exampIe, JP-A No. 8-104830 discloses a method of producing a fine particle for paint characterized in that a polymer polymerization reaction solution in a polymerization process for producing a polymer solid raw material for paint is dissolved in a supercritical phase using carbon dioxide and a polar organic solvent, and expanded rapidly.
Conventionally, polymers such as a fine particle for paint, and the like are produced by a solution polymerization method using a large amount of organic solvent, and the like from the standpoints of control of polymerization reaction speed, handling of a polymerization product, and the like. However, in the solution polymerization method, a polymer is produced in solution condition containing a solvent approximately in half amount, thus, a de-solventing process is necessary of removing a solvent from the resulting polymer solution and drying the polymer, after polymerization, leading to a complicated process. Treatment of an organic solvent vaporizing in the de-solventing process is also problematical.
In contrast, recently, there is a trial for producing a polymer using as a solvent a supercritical fluid, particularly, supercritical carbon dioxide. When supercritical carbon dioxide is used as a solvent, there is no necessity to effect removal of solvent and drying after polymerization, therefore, the process can be simplified and cost can be decreased. From the standpoint of no use of an organic solvent, environmental load is also small. Additionally, carbon dioxide can be easily recovered and recycled as compared with an organic solvent. Further, in many cases, there is a difference in solubility in carbon dioxide between a polymer and a monomer. As a result, when supercritical carbon dioxide is used as a solvent, the amount of unreacted monomers contained in a product polymer lowers, thus, a polymer of higher purity can be produced.
Regarding a method of producing a polymer using a supercritical fluid, Japanese Patent Application National Publication (Laid-Open) No. 7-505429, for example, discloses a method of producing a fluoro polymer containing a process of solubilizing a fluoro monomer in a solvent containing supercritical carbon dioxide and a process of thermally polymerizing a fluoro monomer in the solvent in the presence of a radical polymerization initiator, to produce a fluoro polymer.
JP-A No. 2000-26509 discloses a method of producing a fluoro polymer in which at least one fluorinated monomer is thermally polymerized in supercritical carbon dioxide using dimethyl(2,2′-azobisisobutyrate) as an initiator.
JP-A No. 2002-327003 discloses a method of producing a fluorinated alkyl group-containing polymer in which a radical-polymerizable monomer component containing a fluorinated alkyl group-containing (meth)acrylate in an amount of 20 wt % or more is thermally polymerized using supercritical carbon dioxide as a polymerization solvent.
JP-A No. 2001-151802 discloses a method of producing a polymer fine powder in which a monomer composition containing an ethylenically unsaturated monomer having a carboxyl group such as (meth)acrylic acid and the like is thermally radical-polymerized in supercritical carbon dioxide to give a polymer fine powder.
JP-A No. 2002-179707 discloses a method of producing a polymer fine particle in which a monomer such as methyl methacrylate and the like is thermally polymerized in supercritical carbon dioxide using a radical polymerization initiator which is a polymer having a specific structure substantially soluble in supercritical carbon dioxide.
JP-A No. 2002-128808 discloses a method of producing a polymer in which a polymerizable monomer such as methyl methacrylate, styrene and the like is thermally radical-polymerized in supercritical carbon dioxide in the presence of a specific non-polymerizable dispersing agent such as docosanoic acid, myristic acid and the like.
Kobayashi Masanori et al. “Dispersion polymerization of vinyl monomer using supercritical carbon dioxide” “Shikizai” vol. 75, No. 8, p. 371-377, 2002 describes dispersion polymerization of various acrylic monomers using, as a solvent, supercritical carbon dioxide and using, as a surfactant, poly(1,1,2,2-tetrahydroheptadecafluorodecyl acrylate) and poly(1,1,2,2-tetrahydroheptadecafluorodecyl methacrylate) obtained by a polymerization reaction using supercritical carbon dioxide as a solvent.
As described above, though polymer production methods of thermally polymerizing a monomer in a supercritical fluid such as supercritical carbon dioxide and the like have been previously investigated, there is known no method for producing a polymer in which a monomer is photo-polymerized in a supercritical fluid.